1. Field of the Invention
This invention relates to the preparation of diamines and is more particularly concerned with the selective removal of 2,2'- and 2,4'-diaminodiphenylmethane from mixtures with 4,4'-diaminodiphenylmethane.
2. Description of the Prior Art
The condensation of aniline with formaldehyde in the presence of catalysts such as mineral acids, siliceous materials and the like, is well-known in the art; see, for example, U.S. Pat. Nos. 2,638,730; 2,950,263; 3,260,751; 3,277,173; 3,297,759; 3,362,979; and 3,476,806. The principal component of the product of this condensation is di(aminophenyl)methane the remaining components being oligomeric methylene polyphenyl polyamines, e.g., triamines, tetramines, etc. The proportion of diamine present in the mixture depends to a large extent on the molar proportion of aniline to formaldehyde. In general, the higher the proportion of aniline to formaldehyde, the higher the proportion of diamine in the product. The majority of the diamine is obtained as the 4,4'-isomer, the 2,4'-isomer being present in relatively minor proportion together with very small amounts of the 2,2'-isomer. The proportion of the isomers in any given product is dependent upon the reactant proportions and conditions employed in the reaction.
To date, no process has been devised which will give a product containing diamine which is exclusively in the form of the 4,4'-isomer. Proportions of 4,4'-isomer as high as 98 percent and as low as 40 percent or less have been reported. However, for many purposes, particularly where the diamine is to be used as an intermediate in the preparation of linear polyamides, polyimides, and similar polymers, it is desirable, if not essential, that the diamine be substantially pure 4,4'-isomer, i.e., that the content of 2,2'- and/or 2,4'-isomer be only 2 percent by weight or less. Accordingly, the diamine isolated from the aniline-formaldehyde condensation (or the corresponding diisocyanate obtained by phosgenation of the diamine alone or as part of the mixture of polyamines obtained in the condensation) has been purified by conventional techniques such as fractional distillation, fractional crystallization, and the like, to achieve the desired purity of the 4,4'-isomer. Not only are such techniques tedious and expensive to operate on a commercial scale, but they produce, as by-product, the 2,4'-isomer, or fractions enriched in 2,4'-isomer, which are of much less utility than the 4,4'-isomer.
We have now found that the 2,2'- and 2,4'-diaminodiphenylmethanes can be selectively removed from a mixture of said isomers with 4,4'-diaminodiphenylmethane by a process which will be described below. This process can be applied successfully to relatively pure mixtures of 2,2'-, 2,4'- and 4,4'-diaminodiphenylmethane as well as to mixtures of these diamines with oligomeric polyamines obtained in the condensation of aniline with formaldehyde.